1. Field of the Invention
The present invention relates to the manufacturing of decorative laminates using inkjet technology.
2. Description of the Related Art
Gravure, offset and flexography are being increasingly replaced for different applications by industrial inkjet printing systems, which have now proven their flexibility in use, such as variable data printing making short production runs and personalized products possible, and their enhanced reliability, allowing incorporation into production lines.
Inkjet technology has also caught the attention of manufacturers of decorative surfaces, such as laminate floor. In the state-of-the-art production process for manufacturing decorative panels as shown by FIG. 1, a paper manufacturer (11) supplies a paper roll (12) to a decor printer (13) who uses gravure printing (14) in order to deliver a decor paper roll (16) to a warehouse (17) of a floor laminate manufacturer (20). Some decor printers (13) are now investigating inkjet printing (15) instead of gravure printing. Rotogravure printing on the porous decor paper generally uses ink having a viscosity at 25° C. of 1 to 2 Pa·s, while the viscosity of inkjet inks is much lower, often up to 15 mPa·s at 25° C. The floor laminate manufacturer (20) stores the decor paper rolls (16) having different decorative patterns in his warehouse (17). Depending on the market demand, the floor laminate manufacturer (20) then selects the decor rolls (16) with the desired decorative pattern in his warehouse (17). The selected decor rolls (16) are then impregnated (18) and cut to size (19) for manufacturing ready-to-use floor laminate (21). The warehouse (17) is necessary as a buffer for sudden large market demands of a specific floor laminate because there is a large time delay between ordering and delivering of new decorative paper rolls (16).
An approach to reduce the size of the warehouse and time delays is treated by EP 2431190 A (THEODOR HYMMEN), which discloses in FIG. 1 a method for producing a digitally printed sheet, web or plate-shaped workpiece (20) with wear-resistant surface including the steps of: A) providing a digital data set to a digital printing device (1); B) providing a printable workpiece (20) to the printing apparatus (1); C) digital printing at least an acrylate printing ink (22) on the printable workpiece (20) using the printing apparatus (1) and thereafter supplying a resin mixture (5, 21) to the digitally printed workpiece; and D) curing the resin mixture (5, 21) by means of a heated press (7). The time delay can be avoided by the floor laminate manufacturer incorporating the manufacturing of decorative paper rolls into its own production process. Although acrylate based inks provide excellent image quality, on heat pressing into a decorative laminate adhesion problems are observed between the ink layer and the protective layer containing a thermosetting resin, such as melamine. The cause of these adhesion problems is that the water vapour produced during the polycondensation of the melamine cannot penetrate through the polymerized acrylate ink layer. Paragraph discloses that the adhesion problems are especially seen at high ink coverages of more than 10 g/m2 (e.g. a dark wood laminate).
Aqueous inkjet inks have been used for manufacturing decorative laminates by single pass inkjet printing. For example, in the Palis roll-to-roll single pass printing systems (http://www.palis-digital.com/en/portfolio/sp-drucker.html) aqueous inkjet inks are printed on a paper substrate having a special ink acceptance layer. Printed decorative paper rolls are then delivered to the floor laminate manufacturer, who then impregnates the printed decorative paper with a thermosetting resin before cutting into sheets. Dark wood laminate could be manufactured exhibiting no adhesion problems.
In manufacturing decorative laminate having a wood decorative pattern, often a wood grain is embossed into the decorative laminate during heat pressing. It has been observed that it is very difficult to align the wood grain with the wood decorative pattern due to the dimensional instability of the decor paper by impregnation with thermosetting resin after printing.
EP 2277630 A (VITS) discloses a coating device integrated into an impregnation line that uses a roller pair (3) comprising drivable rollers (3a, 3b) forming an adjustable metering opening for coating a liquid containing abrasion-resistant particles onto an impregnated paper. While the coating quality suffices for applying abrasion-resistant particles, it was found that for the application of an ink acceptance layer the coating quality was too inhomogeneous and caused image artifacts, especially near the ends of the rollers.
Hence, there is still a need for improved manufacturing methods of decorative surfaces using inkjet technology and melamine resin as a wear coating exhibiting good image quality, adhesion and wherein an embossed wood grain corresponds with a decorative pattern.